The present invention relates to cerium-doped alkaline earth scintillators useful for the detection of high-energy radiation. In particular, the present invention relates to a shaped cerium-doped alkaline-earth hafnium oxide scintillator having improved transparency and light output for use in positron emission tomography. The present invention also relates to detectors and detection systems incorporating a scintillator comprising polycrystalline cerium-doped alkaline earth hafnium oxide.
Positron emission tomography (xe2x80x9cPETxe2x80x9d) is a medical imaging technique in which a radioactively labeled substance is administered to a patient and then traced within the patient""s body by means of an instrument that detects the decay of the radioactive isotope. In PET, a chemical tracer compound having a desired biological activity or affinity for a particular organ is labeled with a radioactive isotope that decays by emitting a positron. The emitted positron loses most of its kinetic energy after traveling only a few millimeters in a living tissue. It is then highly susceptible to interaction with an electron, an event that annihilates both particles. The mass of the two particles is converted into 1.02 million electron volts (1.02 MeV) of energy, divided equally between two 511 keV photons (gamma rays). The two photons are emitted simultaneously and travel in almost exactly opposite directions. The two photons penetrate the surrounding tissue, exit the patient""s body, and are absorbed and recorded by photodetectors typically arranged in a circular array. Biological activity within an organ under investigation can be assessed by tracing the source of the radiation emitted from the patient""s body to the photodetectors.
The value of PET as a clinical imaging technique is in large measure dependent upon the performance of the photodetectors. Each photodetector comprises a scintillator cell or pixel coupled to photomultiplier tubes. When a photon generated from an annihilation of the positron strikes a scintillator cell, it excites the scintillator material to produce light that is sensed by the photomultiplier tubes. The electrical signals from the photomultiplier tubes are processed to produce an image of the patient""s organ. The scintillator material desirably has good stopping power, high light output, and fast decay time. Stopping power is the ability to stop the 511 keV photons in as little materials as possible so as to reduce the overall size of the photodetectors and, therefore, enhance the light collection efficiency and energy resolution. Stopping power is typically expressed as the linear attenuation coefficient xcfx84 having units of inverse centimeters (cmxe2x88x921). After a photon beam has traveled a distance x in a scintillator material, the proportion of photons that has not been stopped by the scintillator material is exp(xe2x88x92xcfx84xc2x7x). Thus, for a good scintillator material, xcfx84 should be as large as possible. High light output is important because the photodetectors will have higher sensitivity, and, thus, the dose of the radioactive material administered to the patient can be reduced. Decay time (or also known as time constant, decay constant, or primary speed) is a measure of how fast the scintillator material stops emitting light after a cessation of excitation by the 511 keV photon. Short decay time allows for more rapid scanning, and, thus, better observation of the motion of the body""s organs. Known scintillator materials for PET are thallium-doped sodium iodide (NaI:Tl), cesium fluoride (CsF), barium fluoride (BaF2), and bismuth germanate (Bi4Ge3O12 or xe2x80x9cBGOxe2x80x9d). Each of these scintillator materials has something left to be desired. NaI:Tl has a good stopping power but a long decay constant of about 250 nsec (nanoseconds). CsF has relatively poor stopping power of about 0.43 cmxe2x88x921 and is highly hygroscopic. BGO has a relatively good stopping power but a relatively low light output and a long decay constant of about 300 nsec. Although BaF2 is not as hygroscopic as CsF, it has a poor stopping power similar to that of CsF and a much longer decay constant of about 620 nsec.
Therefore, there is a continued need for scintillator materials for PET that have better properties than those currently available.
The present invention provides improved scintillator compositions comprising alkaline earth hafnium oxide doped with cerium. The scintillator compositions are useful in the detection of high-energy radiation, such as X, xcex2, or xcex3 radiation. Particularly, the scintillators of the present invention have improved light output, short decay time, and high stopping power in positron emission tomography applications. The scintillator compositions of the present invention has a general formula of AHfO3:Ce; wherein A is an alkaline earth metal selected from the group consisting of barium, strontium, calcium, and combinations thereof; and the atomic ratio of A:Hf is from about 0.9:1 to about 1.1:1. In this formula, Ce written after the colon represents the dopant. Ce is present in an amount from about 0.005 atom percent to about 5 atom percent.
In one aspect of the present invention, the alkaline earth metal is partially substituted with at least one trivalent ion or one divalent ion.
In another aspect of the present invention, hafnium is partially substituted with at least one divalent ion, one trivalent ion other than cerium, one other tetravalent ion, or combinations thereof.
According to one aspect of the present invention, a method for producing a scintillator material comprising a polycrystalline alkaline earth hafnium oxide doped with cerium, the scintillator being useful for a detection of X, xcex2, or xcex3 radiation, comprises the steps of: (1) providing amounts of compounds of at least cerium, hafnium, and at least an alkaline earth metal selected from the group consisting of barium, strontium, calcium, and combinations thereof; the amounts of the compounds being selected such that the final composition of the scintillator material is achieved; (2) mixing together the compounds to form a mixture; and (3) firing the mixture at a temperature and for a time sufficient to convert the mixture to an alkaline earth hafnium oxide doped with cerium.
According to another aspect of the present invention, the method further comprises conducting a hot isostatic pressing of powder of the alkaline earth hafnium oxide doped with cerium to form a shaped polycrystalline scintillator that has improved light transparency.
In still another aspect of the present invention, a detector is provided for PET and comprises a polycrystalline scintillator comprising an alkaline earth hafnium oxide doped with cerium having a general formula of AHfO3:Ce; wherein A is at least an alkaline earth metal selected from the group consisting of Ba, Sr, Ca, and combinations thereof; the atomic ratio of A:Hf is from about 0.9:1 to about 1.1:1; and Ce is present in an amount from about 0.005 atom percent to about 5 atom percent.
In still another aspect of the present invention, such a detector is incorporated in a PET system.
Other features and advantages of the present invention will be apparent from a perusal of the following detailed description of the invention and the accompanying drawings in which the same numerals refer to like elements.